In the field of tooth filling materials which are cured by polymerization it was considered to be a great step forward when Rafael L. Bowen introduced long-chain monomeric dimethacrylate re-action products of bisphenol A and its derivatives with glycidyl methacrylate, in particular the so-called bis-GMA; and fine quartz glass powder for reinforcement of the plastic matrix in place of the methyl methacrylate used up to then (U.S. Pat. No. 3,066,112).
A further example of a dental material containing, in addition to organic monomers, a finely divided inorganic filler is described in U.S. Pat. No. 3,539,533. The polymerizable binder in this case is a mixture of bis-GMA, bisphenol A-dimethacrylate, diluted monomers, in particular triethylene glycol dimethacrylate and, if required, methacrylic acid in small amounts which is used, together with approximately 65 to 75 weight-% of the inorganic filler, for example silicon dioxide, glass, aluminum oxide or quartz. The inorganic filler can be of a particle size of approximately 2 to 85 micrometers; for improving the bond between filler and resin/filler is pretreated with silane, for example 3-methacryloyl oxypropyl trimethoxysilane.
Fillings for teeth, caps, artificial teeth and the like, having good mechanical properties, can be produced from dental materials (composites) containing inorganic fillers of the most varied chemical composition—mainly of glass, ceramic materials or glass-ceramic materials which have been treated with silane materials to improve the adhesion between filler and resin.
The use of micro-fine inorganic fillers with average particle sizes between 0.01 to 0.4 micrometers also resulted in dental plastic products which were improved in the esthetic sense. These products could be polished to a high gloss and have a transparency similar to that of natural teeth (DE 24 03 211 C3).
The so-called hybrid materials represent a further step in the development of resin based dental materials which contain micro-fine fillers as well as conventional fillers (macro fillers). Such a dental material is known, for example, from DE 24 05 578 C3. It contains 30 to 80 weight-% of a mixture of amorphous silicic acid produced by means of flame hydrolysis (pyrogenous silicon dioxide) of a maximum particle size of 0.07 micrometers and finely divided glass, preferably boron silicate glass, glass containing barium oxide or lanthanum oxide or lithium aluminum silicate glass of a particle size of up to 5 micrometers.
The dental filler described in DE 34 03 040 C2 contains 60 to 90 weight-% of a filler mixture of 5 to 20 weight-% of a filler opaque to X-rays with a particle size distribution between 0.5 and 40 micrometers, 20 to 35 weight-% of a filler opaque to X-rays with a particle size distribution between 0.2 and 15 micrometers and 45 to 75 weight-% of a silicon dioxide micro-filler with a particle size distribution between 5 and 150 nanometers.
A further example of a hybrid material is the dental material described in EP 382 033 A2 which contains, in addition to polymerizable acrylates or methacrylates and a catalyst for photo-polymerization (photo activator), 5 to 80 weight-% of silanized glass or silanized glass-ceramics with a mean particle size between 0.1 and 10 micrometers and 2 to 10 weight-% of a surface-treated micro-filler. The inorganic fillers used for reinforcing resin based dental materials mostly have a surface treated with a silane, for example 3-methacryloyl oxypropyl trimethoxy silane, which improves the compatibility with the organic components (DE 34 03 040 C2) and causes a chemical adhesion between the filler and the plastic matrix. A further improvement of the filler/plastic bond can be achieved when the possibility of a physical adhesion exists in addition to the chemical adhesion. In accordance with a proposal in U.S. Pat. No. 4,215,033, for example, physical adhesion can be provided by the use of a semi-porous filler obtained by etching a two-phase glass.
U.S. Pat. No. 5,707,440 A describes larger filler particles which are covered by smaller particles of different material. The larger particles have a lower melting point than the small particles and are typically glass particles. Thus when softened in the melting range, the larger particles are covered by the higher melting smaller particles. After cooling the particles keep sticking together and form a special kind of filler particles as illustrated in FIG. 1 of U.S. Pat. No. 5,707,440 A.
However, due to its construction from 2 kinds of inorganic filler with different refractive index, its disadvantage is the limited translucency of resulting composites that is not suitable for aesthetic restorations. The dimension of the SiO2 surface decorated dental glass filler is determined by the central glass particle. Bigger particles of this type can not provide an improved gloss stability in comparison to conventional submicrohybrid composites.
Microfiller composites were developed to perform excellent esthetic properties, based on special pre-polymerized filler particles made from fumed silica (e.g. Aerosil . . . ) and suitable (meth)acrylate crosslinkers (e.g. DCDMA, DDMA, . . . ) by industrial polymerization and grinding. Such microfiller composite materials are appreciated due to their permanent gloss stability. Commonly most successful sub-microhybrid composites and nano-hybrid composites can not provide this advantage. An issue of the microfiller composites is the limited flexural strength below 100 MPa that allows only anterior restorations of class III (and class IV with limitations). Moreover microfiller composites do not provide any radio-opacity due to the filler load of fumed silica only.
A new filler technology was introduced with FILTEK by 3M-ESPE using agglomerated nanoparticles prepared by a thermal procedure probably. The resulting agglomerated filler particles are softer than compact dental glass fillers with a comparable size. The FILTEK fillers mimic the construction of fumed silica, which is also build of silica nano-agglomerates, but FILTEK fillers are constructed stronger from SiO2 (and ZrO2 to provide a radio-opacity) and improved mechanical performance of the composite. A higher content of Zirconia is necessary to increase the level of radio-opacity but on the other hand brings the disadvantage to reduce translucency at the same time.